ISDN (Integrated Services Digital Network) is an all digital communications network. The basic rate ISDN line is a user-to-network interface consisting of three communications channels. There are two B channels for voice, data and facsimiles and one D channel for signaling. Devices known as terminal adapters enable the user to interface various digital and analog communications equipment, such as computers, phones, facsimile machines, and analog modems to the ISDN line.
The two B channels, data channels, have different telephone numbers or directory numbers. Each channel is assigned a directory number by the local telephone company and the numbers are programmed into the terminal adapter when the adapter is installed and configured. Accordingly, the terminal adapter is capable of independently placing and receiving calls over both of the B channels.
Configuration also involves setting other parameters, such as the switch type, the switch version, the terminal endpoint identifiers (TEls) and the service profile identifiers (SPIDs) in addition to the directory numbers. After the configuration parameters are set, the terminal adapter attempts to make a connection to the ISDN line by establishing several protocol layers. However, the directory numbers are not involved in those protocol layers and if the terminal adapter has stored therein an incorrect directory number associated with a channel, it will still be able to establish the basic protocol layers and place outgoing calls on the channel, yet it will not be able to receive incoming calls on the channel.
With most terminal adapters, if a wrong directory number is programmed into the terminal adapter during configuration, this will not become apparent until the user is unable to receive a call to the terminal adapter over the channel with the incorrect directory number. Even then it is unlikely that the user will determine that the incorrect directory number is causing the problem, especially when the user has correctly entered the wrong directory number supplied by the telephone company. Generally, the user will have to contact the terminal adapter manufacturer who will then determine the cause of the problem and refer the user to the telephone company to obtain the proper directory number.
One terminal adapter, the U.S. Robotics Sportster ISDN 128K terminal adapter, performs a "self call" test after configuration. With this test the terminal adapter attempts to place a call from one channel of the terminal adapter and receive that call on the other channel of the terminal adapter. If the call is not received, the terminal adapter indicates to the user that the terminal adapter appears to be connected to the ISDN network, and that the failed call may be due to one or more of a number of problems. The problems specified by the terminal adapter are that the ISDN drivers may be locked in a bad state, that the telephone company settings are incorrect (i.e. wrong switch type, phone number(s), SPID(s), or local access number), that the current dialing location information is incorrect or that the ISDN line is only provisioned for a single data channel. This terminal adapter provides only general information to the user as to why the self call test could possibly have failed. It does not specifically inform the user when the directory numbers are invalid and therefore it is not a very useful diagnostic tool.
Thus, there exists a need for an apparatus for and method of specifically checking the validity of directory numbers for a device for interfacing communications equipment to a telephone line, such as an ISDN terminal adapter and such an apparatus for and method of clearly indicating to the user the validity or invalidity of such directory numbers.